Metal oxide films are useful in a variety of applications in the semiconductor industry such as, for example, lithographic hardmasks, underlayers for anti-reflective coatings and electro-optical devices.
As an example, photoresist compositions are used in microlithography processes for making miniaturized electronic components such as in the fabrication of computer chips and integrated circuits. Generally, a thin coating of a photoresist composition is applied to a substrate, such as a silicon based wafer used for making integrated circuits. The coated substrate is then baked to remove a desired amount of solvent from the photoresist. The baked coated surface of the substrate is then image-wise exposed to actinic radiation, such as, visible, ultraviolet, extreme ultraviolet, electron beam, particle beam and X-ray radiation.
The radiation causes a chemical transformation in the exposed areas of the photoresist. The exposed coating is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the photoresist.
The trend towards the miniaturization of semiconductor devices has led to the use of new photoresists that are sensitive to shorter and shorter wavelengths of radiation and has also led to the use of sophisticated multilevel systems to overcome difficulties associated with such miniaturization.
Absorbing antireflective coatings and underlayers in photolithography are used to diminish problems that result from radiation that reflects from substrates which often are highly reflective. Reflected radiation results in thin film interference effects and reflective notching. Thin film interference, or standing waves, result in changes in critical line width dimensions caused by variations in the total light intensity in the photoresist film as the thickness of the photoresist changes. Interference of reflected and incident exposure radiation can cause standing wave effects that distort the uniformity of the radiation through the thickness. Reflective notching becomes severe as the photoresist is patterned over reflective substrates containing topographical features, which scatter light through the photoresist film, leading to line width variations, and in the extreme case, forming regions with complete loss desired dimensions. An antireflective coating film coated beneath a photoresist and above a reflective substrate provides significant improvement in lithographic performance of the photoresist. Typically, the bottom antireflective coating is applied on the substrate and cured followed by application of a layer of photoresist. The photoresist is imagewise exposed and developed. The antireflective coating in the exposed area is then typically dry etched using various etching gases, and the photoresist pattern is thus transferred to the substrate.
Underlayers containing high amount of refractory elements can be used as hard masks as well as antireflective coating. Hard masks are useful when the overlying photoresist is not capable of providing high enough resistance to dry etching that is used to transfer the image into the underlying semiconductor substrate. In such circumstances a material called a hard mask whose etch resistance is high enough to transfer any patterns created over it into the underlying semiconductor substrate. This is made possible because the organic photoresist is different than the underlying hard mask and it is possible to find an etch gas mixture which will allow the transfer of the image in the photoresist into the underlying hard mask. This patterned hard mask can then be used with appropriate etch conditions and gas mixtures to transfer the image from the hard mask into the semiconductor substrate, a task which the photoresist by itself with a single etch process could not have accomplished.
Multiple antireflective layers and underlayers are being used in new lithographic techniques. In cases where the photoresist does not provide sufficient dry etch resistance, underlayers and/or antireflective coatings for the photoresist that act as a hard mask and are highly etch resistant during substrate etching are preferred. One approach has been to incorporate silicon, titanium or other metallic materials into a layer beneath the organic photoresist layer. Additionally, another high carbon content antireflective or mask layer may be placed beneath the metal containing antireflective layer, such as a trilayer of high carbon film/hardmask film/photoresist is used to improve the lithographic performance of the imaging process. Conventional hard masks can be applied by chemical vapor deposition, such as sputtering. However, the relative simplicity of spin coating versus the aforementioned conventional approaches makes the development of a new spin-on hard mask or antireflective coating with high concentration of metallic materials in the film very desirable.
Underlayer compositions for semiconductor applications containing metal oxides have been shown to provide dry etch resistance as well as antireflective properties. Conventional soluble metal compounds to form metal oxide films, such as metal alkoxides, however, have been found to be very unstable to moisture in air creating a variety of issues, including shelf life stability, coating problems and performance shortcomings. Metal oxides have solubility problems in solvents typically used and accepted in the semiconductor industry. Thus there is an outstanding need to prepare spin-on hardmask, antireflective coatings and other underlayers that contain organic solvents soluble, stable metal compounds even after exposure to air, and which can also be strippable in chemical solutions after curing of the film to form metal oxide.